1. Field of the Invention
The present invention relates to an immunotoxin. The invention further relates to a method of treating T cell leukemias or lymphomas, graft-versus-host diseases, cancer, and autoimmune diseases by administering an immunotoxin.
2. Background
Immunotoxins are toxins with altered receptor specificities. The alteration is achieved by coupling a monoclonal antibody (mAb) or growth factor to the toxin or toxin fragment. Plant and bacterial protein toxins intoxicate cells by a multi-step process whereby different toxin domains sequentially interact with cellular components. The intoxication pathway at a minimum consists of surface receptor binding, toxin processing, intracellular routing of toxin A chains to the cytosol, and enzymatic inactivation of protein synthesis (Neville and Hudson (1986) Ann. Rev. Biochem. 55:195). The goal of immunotoxin research has been to achieve targeted cell killing comparable to the enormous but indiscriminate cell killing power of the native toxins. An equally important goal has been to maintain the low non-target cell toxicity of toxin A chains, which lack cell receptor binding and membrane translocation functions (Youle and Neville (1982) J. Clin. Biol. 257:1598; Neville (1986) in CRC Crit. Rev., Therap. Drug Carrier Syst., CRC Press Inc., 2:329; Immunotoxins, Frankel ed. (1988) Kluwer Academic Publishers). Because of this latter consideration most in vivo clinical studies have focused on A chain immunotoxins or immunotoxins with truncated B chains lacking the receptor binding domain. While some clinical results have been encouraging, the reproducible achievement of both goals is at present uncertain (Program and Abstracts 2nd Int. Symposium on Immunotoxins, June 1990, Lake Buena Vista, Fla.).
Recently, Youle and co-workers have introduced highly efficacious holo-immunotoxins based on diphtheria toxin (DT) binding mutants (Greenfield et al. (1987) Science 238:536; Johnson et al. (1988) J. Biol. Chem. 263:1295; Johnson et al. (1989) J. Neurosurg. 70:240). These DT binding site mutants were equal to the wild-type immunotoxins in potency when directed at the human transferrin receptor (TFR) or human CD3, a component of the T cell receptor complex. Since the binding of the mutants was only 1/100-1/1000 of native DT, the toxin receptor appeared to be not needed along the intoxication pathway. This conclusion is limited to immunotoxins which route through CD3 and TFR, because similar immunotoxins directed at CD5 and the high-molecular weight-melanoma-associated antigen are relatively non-toxic (Neville et al. (1989) J. Biol. Chem. 264:14653). On the basis of data obtained with acid-cleavable conjugates which released free DT or the DT binding site mutant CRM9 in acidified endosomes, it was concluded that the DT receptor participates in the optimal intracellular routing of DT and many DT conjugates (Neville et al. (1989) J. Biol. Chem. 264:14653). It was also concluded that CD3 and TFR can perform the same routing function as the DT receptor, thus obviating the requirement of a DT receptor interaction for the binding site mutant conjugates anti-CD3-CRM9 and TFR-CRM9 (Intracellular routing of ricin based immunotoxins via the ricin receptor leading to enhanced efficacy has also been reported. Youle et al. (1981) Cell 23:551; Marsh and Neville (1986) Biochem. 25:4461; Youle and Colombatti (1987) J. Biol. Chem. 262:4676). The disadvantages of these chemically conjugated immunotoxins are linkage heterogeneity, Fc receptor interactions and a mutated toxin-binding domain in which binding to DT receptors is reduced but not eliminated. These disadvantages were overcome by developing recombinant immunotoxins based on diphtheria toxin truncated at residue 390 which provided optimal translocation of the toxin A chain into the cytosol while eliminating the toxin binding domain (Williams et al., 1990 and Thompson et al., 2001). These DT based immunotoxins were restricted to having the antibody moiety placed C-terminal to the truncated toxin because antibody domains fused to the N-terminal of the toxin interfered with translocation of biologically active A chain (Madshus et al., 1992 and Hexam et al., 2001).